1. Field of the Invention
This invention relates to an oil hydraulic circuit for a hydraulic shovel.
2. Descritpion of the Prior Art
An oil hydraulic circuit of the type wherein flows of discharge pressure oil from a pair of hydraulic pumps are supplied separately to a pair of hydraulic directional control valve sets each composed of a series of hydraulic directional control valves and constituting a parallel circuit, and when a first actuator is to operate at a low speed, pressure oil from a hydraulic directional control valve belonging to a first one of the hydraulic directional control valve sets is supplied to the actuator, but when the first actuator is to operate at a high speed, pressure oil from another hydraulic directional control valve belonging to the other or second hydraulic directional control valve set is joined to the pressure oil from the hydraulic directional control valve belonging to the first hydraulic directional control valve set and is supplied to the actuator. However, operating conditions of a hydraulic shovel are normally very complicated and it often occurs that it is desired, during operation of the first actuator at a high speed, to cause a second actuator connected to a hydraulic directional control valve belonging to the second hydraulic directional control valve set to operate at the same time and additionally to hold the relative operating speeds of the first and second actuators at a predetermined ratio in accordance with current operating conditions. However, various load pressures are required for the actuators, but the load pressures are restricted to an operating pressure of one of the actuators which presents a lower load pressure. Thus, the lower load pressure side actuator operates in a preceding relationship to the other actuator, and a starting requirement for the other actuator cannot be met. Accordingly, operability for a smooth composite operation cannot be obtained.
When a composite operation is performed wherein operating levers for the hydraulic directional control valves are caused to operate at the same time, the opening of a passage of the hydraulic directional control valve belonging to the first hydraulic directional control valve set upon changing over when the first actuator is to operate at a high speed is conventionally limited to a fixed value, or the amounts of operation of the operating levers are adjusted delicately, to cope with the condition described above.
This will be described below in connection with an example with reference to FIG. 3 which shows an oil hydraulic circuit for a common hydraulic shovel C shown in FIGS. 4 and 5. It is to be noted that the first actuator described above corresponds to a hydraulic cylinder 13 for lifting a working device and the second actuator corresponds to a hydraulic unit 14 for turning an upper turning body D.
The oil hydraulic circuit shown in FIG. 3 incudes a first hydraulic directional control valve set A which consists of hydrualic directional control valves 5, 6 and 7, a cut-off valve 11 and so forth. Pressure oil is supplied to the hydraulic directional control valve set A by way of a pipe line 18 from one of a pair of main pumps (not shown). The hydraulic directional control valve 5 is positioned on the most upstream side and is provided for exclusive use for an actuator for running. Thus, pressure oil is preferentially supplied to the hydraulic directional control valve 5. The hydraulic directional control valves 6 and 7 are located on the downstream side of the hydraulic directional control valve 5 and connected in a parallel circuit. The oil hydraulic circuit further includes another second hydraulic directional control valve set B consisting, similarly to the first hydraulic direction control valve set A described above, of hydraulic directional control valves 8, 9 and two-way throttling valve 20, a cut-off valve 12 and so forth. Pressure oil from the other main pump (not shown) is supplied to the second hydraulic directional control valve set B by way of another pipe line 19.
In the oil hydraulic circuit, when the hydraulic cylinder 13 normally called a boom cylinder for lifting a working device is to be extended, a pilot pressure of P.sub.B1 is caused to act upon a pilot oil chamber 6a of the hydraulic directional control valve 6, and when a composite operation is to be performed wherein an upper turning body D is turned leftwardly or rightwardly at the same time with such lifting motion for the working device, the pilot pressure P.sub.R1 is also caused to act upon a pilot oil chamber 14a or 14b of the hydraulic unit 14 for turning motion. However, when the value of the pilot pressure P.sub.B1 is comparatively low, that is, when the working device is being lifted slowly, the two-way throttling for providing restrictive flow and for cutting-off flow valve 20 does not operate while maintaining its position G shown in FIG. 3, and only the hydraulic directional control valve 6 and the hydraulic unit 14 for turning motion operate. Thus, since pressure oil flows from the individually different hydraulic pressure sources are supplied individually to the hydraulic cylinder 13 and the hydraulic unit 14, they operate independently of each other. However, when the pressure of P.sub.B1 is increased in order to expand the hydraulic cylinder 13 at a high speed, the two-way throttling valve 20 is simultaneously changed over from its G position to its H position. Consequently, pressure oil of the pipe line 19 flows not only to the hydraulic unit 14 for turning motion but also to the hydraulic cylinder 13 by way of an H position passage of the two-way throttling valve 20.
Consequently, the operating pressure of the pipe line 19 becomes equal to the pressure of one of the hydraulic cylinder 13 and the hydraulic unit 14 which is lower in load pressure than the other.
Generally , in such composite operation of the hydraulic shovel as described above, the pressure upon expansion of the hydraulic cylinder 13 is lower than a pressure required for the hydraulic unit 14 upon starting of the turning motion. Consequently, the latter pressure is limited by the former pressure, and the accelerating performance of the turning body D, that is, the rising characteristic, is deteriorated. Accordingly, the opening area of the H positon passage of the two-way throttling valve 20 is limited as illustrated in FIG. 3 to obtain a throttle effect so that the operating pressure upon the hydraulic unit 14 for turning motion may not become excessively low.
It is to be noted that the cut-off valves 11 and 12 may be required or not required depending upon various hydraulic circuits, but in the present example, when pressure oil from the two main pumps is joined together or else branched to utilize the pressure oil most reasonably in order to realize the linearity of running in a single operation or in a composite operation for running, for operation of the working device, for operation for turning motion or the like, and also realize the rapidity and the certainty of various operations, the cut-off valves 11 and 12 play a role of preventing such pressure oil from flowing out to a tank 21 in vain. However, illustration of an acting circuit for a signal pressure to the pilot oil chambers of the cut-off valves 11 and 21 is omitted in FIG. 3.
Here, a case wherein excavating and loading operations are performed with the hydraulic shovel C described above is described as an example. In particular, after an excavating operation is completed, the hydraulic cylinder 13 is expanded to lift the working device while at the same time the upper turning body D is turned until the working device comes to a suitable vertical postiton above a dump truck E where excavated earth and sand is discharged into the dump truck E. Such sequence of operations is repeated. In this instance, the opening of the H position passage of the two-way throttling valve 20 is determined such that pressure oil of the pipe line 19 may be distrubuted mainly so that expansion of the hydraulic cylinder 13 and operation of the hydraulic unit 14 may be performed at the same time and in addition the lifting height of the working device and a desired turning angle may be reached at the same time.
For example, when the upper turning body D is to be turned by about 90 degrees to perform loading in such an arrangement of the hydraulic shovel C and the dump truch E as shown in FIG. 4. if lifting of the working device and turning motion of the upper turning body D are started at the same time after completion of an excavating operation, then if it is assumed that the opening of the H position passage is fixed such that the vertical position of the working device and the turning angle of the upper turning body D may provide a best loading position with respect to the dump truck E, then in such a loading operation after turning motion of about 180 degrees as shown in FIG. 5, the turning angle is insufficient with respect to the lifting amount of the working device. On the contrary, if an optimum H position opening is provided in such an operation arrangement as shown in FIG. 5, then a disadvantage will take place in such an operation arrangement as shown in FIG. 4. Accordingly, the operating levers must be adjusted delicately each time, and therefore, very high skill is required for rapid and smooth excavating and loading operations.